1. Field of the Invention
This invention relates generally to interpenetrating polymer networks (IPNs) and semi-interpenetrating polymer networks (semi-IPNs). It relates particularly to a process for controlling the degree of phase separation and improving toughness, microcracking resistance and thermal-mechanical performance of high temperature IPNs or semi-IPNs.
2. Description of the Related Art
Highly crosslinked polyimides, such as PMR-15 and bismaleimides (BMIs), which are commercially available, are inherently brittle and prone to microcracking in thermally-cycled fiber composites. A successful way to improve the fracture toughness of these materials has long been sought.
One possible method for improving the fracture toughness of highly crosslinked polyimides is similar to the rubber toughening of epoxies. In this method, a tough minor polymer constituent is dispersed in the brittle thermosetting polyimide resin. (C. B. Bucknell, Toughened Plastics, Applied Sci. Pubs., London (1977)). For polyimide fiber composites, however, this approach has a significant drawback. An attractive feature of polyimides is that they have higher glass transition temperatures (T.sub.g) than the epoxies they replace. The addition of a minor rubber phase would decrease the T.sub.g and the use temperature of the polymer system, because the lower T.sub.g of a rubber phase is responsible for the material's elevated temperature mechanical properties.
Moreover, a tough neat resin produced by conventional toughening methods is not sufficient for producing a useful tough composite, because of the presence of an additional characteristic dimension when a neat resin is used as the matrix in a composite that is nominally one-third resin and two-thirds fiber. In nonunidirectional composites, the resin is divided into essentially individual volumes of resin divided by fibers. A characteristic dimension for such a volume of material can be defined, which then implies a smaller characteristic dimension for the dispersion of the minor constituent used to toughen the resin on this dimension scale in the composite. Thus, it has been established that the micro-mechanical deformation of the resin in the composite must be altered on a dimensional scale that is much smaller than the diameter of the fiber. Consequently, the morphology of the resin in the composite must be controlled according to this second dimensional constraint. For example, to achieve a toughened BMI by the epoxy method, the nominal size of the dispersed rubber phase will be in the range of two to six microns. If this toughened BMI is used to make a composite that is 60 to 70 percent by volume of fibers that are six to nine microns in diameter, a geometric paradox develops. The fiber volume fraction and fiber diameter impose a morphological constraint on the minor phase of the composite matrix.
It is therefore highly desirable to provide a reliable method of controlling the degree of phase separation and, at the same time, improving the fracture toughness, microcracking resistance and thermal-mechanical performance of a high performance IPN or semi-IPN. To control the morphology, the thermodynamic criterion for the mixing of two or more components is that the free energy of mixing, .DELTA.F must be negative. The free energy of mixing is the sum of their heat of mixing, .DELTA.H, and the entropy of mixing, .DELTA.S: EQU .DELTA.F=.DELTA.H-T.DELTA.S
where T is the absolute temperature of mixing. Since any molecule is more attracted to similar than to dissimilar molecules, .DELTA.H is usually endothermic and positive when mixing small molecules. The increase in degree of freedom (or .delta. randomness) and entropy is very high and can easily outweigh the positive heat of mixing, favoring negative .DELTA.F and thus miscibility. However, when mixing large polymer molecules, the thousands of atoms in each molecule must remain together, so that mixing cannot be as random and the gain in .DELTA.S is not nearly as high. Thus, it is seldom possible to outweigh the positive .DELTA.H and very few pairs of chemically dissimilar polymers are miscible to form a homogeneous, single-phase polymer blend. However, in certain cases, miscibility and homogeneity do occur, as a result of specific interactions between the polymer molecules, such as hydrogen-bonding, dipole-dipole interaction and complex formation.
Besides the thermodynamic parameters, the morphology of a polymer blend comprising two or more chemically dissimilar polymeric components can also be kinetically controlled by a freeze-drying process as described below. Polymers in solutions incessantly change their position randomly by thermal agitation. This Brownian motion dominates time-dependent phenomenon in solutions, such as diffusion: small particles placed in a certain point will spread out in time. This effect of the of Brownian motion can be expressed by the Einstein relation: ##EQU1## The Einstein relation states that the diffusion D, which characterizes the thermal motion, is related to the quantity .zeta., which specifies the response to the external force. The constant .zeta. is the friction constant and its inverse 1/.zeta. is called the mobility. The degree of phase separation (or diffusion) of particles in a homogeneous solution is directly related to the Brownian motion of the particles. Accordingly, a minimum separation of the particles can be achieved, when the solvent is removed from the solution under a condition where the level of the Brownian motion of the particles is kept at a minimum.
Freeze-drying is a well-known process used in the food industry to preserve of food by rapidly freezing and removing water from food in a frozen state under high vacuum. Such a process has also been used to prepare various polymeric materials. U.S. Pat. Nos. disclosing the use of a freeze-drying process include: 4,845,162 for making a phenolic graft copolymer; 4,302,553 for preparing an IPN; 3,849,350 for providing a low density syntactic foam; 3,812,224 for making, a porous polymeric material; 4,845,162 for preparing a polymer blend and for making 3,702,779 coated paper.
Freeze-drying processing conditions can be varied significantly, depending on cost and performance requirements, the nature of the products and the type of solvents used. The solvents disclosed in the prior-art for the freeze-drying processes include phenol, formic acid, m-cresol, trichloroacetic acid, chlorophenol, dimethylphenol, trifluroethanol, (U.S. Pat. No. 4,845,162). These protic solvents contain relatively mobile protons and readily form hydrogen-bonding with a polyimide, monomeric reactants used in the preparation of the polyimide or an oligomeric precursor of the polyimide and a polyamic acid which is an polymeric precursor of a polyimide. The hydrogen-bonding between the solvent and soluate makes complete solvent removal exceedingly difficult. If the solvent is not completely removed after a freeze-drying step, trace amounts of the residual solvent can lead to several adverse effects during the final stage of curing a high performance polymer blend. One adverse effect is an increase in phase separation of constituent polymeric components due to enhanced fluidity of the polymer melt induced by the residual solvent. Another is a slowdown in the rate of curing, because additional energy is required to break down the solvent-reactants interaction. If the residual solvent still remains after the final stage of curing it can plasticize the product, lowering the T.sub.g and, thus, elevated temperature mechanical performance.
Besides protic solvents, benzene also has been disclosed for use in a freeze drying process (U.S. Pat. Nos. 3,849,350 and 3,812,224). Most high performance polyimides and their monomeric, oligomeric and polymeric precursors are not readily soluble in benzene. Also, benzene is highly toxic (a carcinogen) and its use in laboratory or manufacture plant is severely restricted by OSHA. Accordingly, there is a continuous search in the art for a process that can control the morphology and improve the thermal-mechanical performance of a high performance polymer blend.
It is an object of the present invention to provide a process for simultaneously controlling the morphology and improving the thermal-mechanical properties of a high performance IPN or semi-IPN. Another object of the present invention is to provide a high performance IPN or semi-IPN that exhibits significantly higher T.sub.g, elevated temperature mechanical properties and thermo-oxidative stability, compared to the state-of-the-art material. Finally, it is an additional object of the present invention to provide a process for the production of IPNs and semi-IPNs useful as adhesives, moldings, and composite matrices.